linux-kernel - Re: [PATCH v2 3/3] vfs: Use per-cpu list for superblock's inode list

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Date:	Mon, 22 Feb 2016 14:04:35 +0100
From:	Jan Kara <jack@...e.cz>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
	Waiman Long <Waiman.Long@....com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Chinner <dchinner@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v2 3/3] vfs: Use per-cpu list for superblock's inode list

On Mon 22-02-16 13:12:22, Peter Zijlstra wrote:
> On Mon, Feb 22, 2016 at 12:54:35PM +0100, Jan Kara wrote:
> > > Also, I think fsnotify_unmount_inodes() (as per mainline) is missing a
> > > final iput(need_iput) at the very end, but I could be mistaken, that
> > > code hurts my brain.
> > 
> > I think the code is actually correct since need_iput contains "inode
> > further in the list than the current inode". Thus we will always go though
> > another iteration of the loop which will drop the reference. And inode
> > cannot change state to I_FREEING or I_WILL_FREE because we hold inode
> > reference. But it is subtle as hell so I agree that code needs rewrite.
> 
> So while talking to dchinner, he doubted fsnotify will actually remove
> inodes from the sb-list, but wasn't sure and too tired to check now.
> 
> (I got lost in the fsnotify code real quick and gave up, for I was
> mostly trying to make a point that we don't need the CPP magic and can
> do with 'readable' code).
> 
> If it doesn't, it doesn't need to do this extra special magic dance and
> can use the 'normal' iterator pattern used in all the other functions,
> greatly reducing complexity.

Yeah, that would be nice. But fsnotify code needs to iterate over all
inodes, drop sb_list_lock and do some fsnotify magic with the inode which
is not substantial for our discussion. Now that fsnotify magic may actually
drop all the remaining inode references so once we drop our reference
pinning the inode, it can just disappear. We don't want to restart the scan
for each inode we have to process so that is the reason why we play ugly
tricks with pinning the next inode in the list.

But I agree it should be possible to just use list_for_each_entry() instead
of list_for_each_entry_safe() and keep current inode pinned till the next
iteration to make it stick in the sb->s_inodes list. That would make the
iteration more standard. Lightly tested patch attached.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

View attachment "0001-fsnotify-Simplify-inode-iteration-on-umount.patch" of type "text/x-patch" (2802 bytes)